[Arran]

The problem with using a pi filter as opposed to a LC filter is that the resistor behaves the same way to AC as it does to DC current, also the higher the current load on the B+ the greater the voltage drop. With a filter choke the choke will pass DC current easily but resists AC current or any voltage fluctuations. For example a 10 henry choke with a DC resistance of 150 ohms blocks acts the same way as a 3700 resistor does to AC ripple, so it will filter better then a 500 ohm resistor but not cut the DC voltage anymore then a 150 ohm resistor under load. In something like a five tube AC/DC set it doesn't make as much difference but in something like a 12 tube console, with a push pull power output stage, it definitely will and should not be considered as a permanent repair.
As to why your volume may be low I would check the plate resistor on the 1st audio tube, these can go open or increase greatly in value cutting down on, or cutting off, the plate voltage going to that tube. It could also be as something as simple as needing an IF alignment, I can't tell you how many sets I have run into where someone fiddled with the IF trimmers and messed it up.
Regards
Arran